1. Field of the invention
The technical domain concerned by this invention relates to quantic effect devices making use of a Coulomb blockade phenomenon. In particular, it is aimed at memory and logical applications of very high density integrated circuits made on a silicon substrate.
2. Discussion of the Background
For example, the principle and feasibility of this type of device are described in the article by K. Yano et al entitled xe2x80x9cRoom-Temperature Single Electron Memoryxe2x80x9d published in IEEE Transactions on Electron Devices, vol 41, No. 9, p 1628-1638, 1994.
The Coulomb blockade phenomenon occurs in conducting islands that are dielectrically insulated from their environment and slightly coupled to it by a tunnel effect. The use of this phenomenon at temperatures close to ambient temperature requires that the total capacitance of each island is of the order of one atto farad which, together with the existence of a tunnel current that is not too low, imposes that the dimensions of the islands must be of the order of one nanometer.
Conducting islands with a characteristic dimension exceeding 10 nm have already been manufactured using conventional lithoetching processes at their limits. However, the scale of a few nanometers cannot be achieved with the current resolution of this type of process.
One solution used consists of making a granular deposition of the conducting material on an insulating layer, for example using a CVD technique or by evaporation; for example, this is described in the article by W. Chen et al. entitled xe2x80x9cCoulomb blockade at 77K in nanoscale metallic islands in a lateral nanostructurexe2x80x9d published in Applied Physics Letters, vol. 66, No. 24, p 3383-3384, 1995.
The area in which the islands will finally be located (wire or rectangle) may be delimited by a lithoetching process (for example lift off). The structure obtained is shown diagrammatically in FIG. 1. In this figure, references 2 and 4 denote two electron reservoirs (for example source and drain of a transistor) between which the nanometric islands 6 are distributed.
The major disadvantage of this approach is that the islands 6 are located at random, which gives strong dispersion of tunnel current levels in different islands. The global current between two electrodes placed on each side of an area of islands is then extremely sensitive to the density of islands, and the characteristics of this current are very dispersed in different samples.
The purpose of this invention is to manufacture a set of islands capable of improving the uniformity of the characteristics, and particularly the current, compared with sets of islands obtained by conventional processes.
The purpose of the invention is a quantic effect device that operates making use of the Coulomb blockade phenomenon, comprising:
a first and a second electron reservoir,
at least one first and one second set of islands separated by a dielectric layer,
a protective insulating layer and a control grid.
Another purpose of the invention is a transistor comprising a source, a drain and a grid, and a channel connecting the source and the drain, a first and a second set of islands separated from each other by a dielectric layer, the said islands being distributed between the channel and the grid, a protective insulating layer covering the two sets of islands.
Consequently, according to the invention, the space between grains is partially filled by covering them with an insulating layer (or a tunnel insulating layer which fixes the current level (tunnel effect)), preferably uniform and with a controlled thickness, and the remaining space is filled by a second deposition of grains. Therefore, successive depositions of islands only form one layer of islands.
If the distance between the first grains, covered by the tunnel insulating layer, is of the same order of magnitude as the grain size, two successive depositions of grains are sufficient to obtain satisfactorily uniform electrical properties.
If the distance between grains is greater than the grain size, several successive depositions of grains may be made, each being covered by a tunnel insulating layer (except for the last layer of grains). Preferably, the various tunnel insulating layers are of the same thickness.
Therefore the invention relates to a microelectronic device comprising a source and a drain, and n successive layers of islands or grains, where nxe2x89xa72, each layer (except for the last) being covered by a tunnel insulating layer. A dielectric layer and a grid control this microelectronic device.
The invention can improve the electrical characteristics of conducting islands or grains located between reservoirs of electrons in the quantic effect device, or between the source and drain of the transistor or the microelectronic device.
The proposed invention can improve the uniformity of the thickness of the dielectric separating the islands and therefore the uniformity of the tunnel current, although the positioning of the islands is still random.
In a quantic effect device according to the invention, the level of the current tunnel varies exponentially with the thickness of the barrier concerned. Making the dielectric surrounding the islands uniform can improve the tunnel current.
Therefore the structure according to the invention (quantic effect device operating based on the Coulomb blockade phenomenon, or a transistor or microelectronic device) comprises a set of islands or grains separated from each other by an insulation, preferably with good control over the thickness, which improves the uniformity of the tunnel current between grains. Good uniformity of the tunnel current is obtained if the thickness of the insulation is controlled.
Another purpose of the invention is a memory device comprising a device or transistor like that described above.
Another purpose of the invention is a process for making a microelectronic device or a transistor or a quantic effect device that operates making use of a Coulomb blockade phenomenon, or a memory device comprising the following steps:
a) formation of a first and a second electron reservoir,
b) formation of a first set of conducting grains or islands,
c) formation of a dielectric layer above the first set of grains or islands,
d) formation of another set of conducting islands or grains, above the dielectric layer.
Steps c) and d) may be repeated N times.
The assembly can then be covered with a protective insulating layer and a control grid.